1. Field of the Invention
This invention relates generally to lids for containers. More particularly, the present invention relates to a lid device of the type which comprises a metallic lid body fittable to an open mouth of a wide-mouthed container and having an inner margin defining an opening, and a closure film attached to the lid body for closing the opening. The invention also relates to a method for making such a lid device.
2. Description of the Prior Art
In general, a lid device of the above-described type is made by forming a metallic plate of e.g. aluminum or tin into a predetermined shape to provide a lid body with an opening, and directly attaching a closure film ( e.g. aluminum foil or resin film) to the lid body for closing the opening. In use, the closure film is peeled off the closure film to provide access into the container.
Typically, the closure film is attached to the lid body by utilizing the heat sealing method, as disclosed in U.S. Patent to Bloeck et al or Japanese Utility Model Application Laid-open No. 62-108229 (Inventors: Susumu TAKADA et al.). Specifically, according to the known heat sealing method, a bonding layer of thermally meltable adhesive or thermoplastic resin is preliminarily formed on the lid body and/or the closure film, and a hot plate is pressed against the closure film overlapped on the lid body to cause thermal fusion of the bonding layer.
The heat sealing method provides uniform attachment of the film over the entire circumference thereof as long as the lid body has a strictly flat attachment surface. In reality, however, it is difficult to make the attachment surface of the lid body strictly flat because the lid body is subjected to various forces at the time of forming from a metallic plate. Thus, the hot plate for heat sealing may pressed unevenly circumferentially of the film, which results in uneven attachment of the film. In an extreme case, the film may be left non-attached at one or more circumferential positions, which causes unexpected gas entry and/or leakage at the non-attached positions.